Add general SDK page to docs

[mr-zwets]

The "compiler" menu section has a general compiler page, similarly I think the SDK menu section should have a general SDK page

it would be a great place to talk about the features of our typescript integration and how the SDK can run front-end and backend

the page could talk about the 4 big classes in the SDK Contract, TransactionBuilder, SignatureTemplate and NetworkProvider

The section could also explain how the CashScript SDK is really a high level Libauth wrapper tightly integrated to support CashScript artifacts
but it could explain that even for non-cashscript-artifacts the library is still very handy

:::tip
You can create an Artifact for a fully hand-written contract so it becomes possible to use the contract with the nice features of the CashScript SDK! An example of this is Cauldron_Swap_Test which uses Artifact bytecode not produced by cashc at all but still uses the CashScript SDK.
:::

it would also be a great place to put things like this

:::info
CashScript offers a TypeScript SDK, which can also be used easily in vanilla JavaScript codebases.
Because of the separation of the compiler and the SDK, CashScript contracts can be integrated into other languages in the future.
:::

It could also be the place in the future where we link to CashScript-py

[mr-zwets]

consider together with #382